The invention relates to a lubricant pump, in particular for a combustion engine of a motor vehicle, having a housing in which at least one pair of intermeshing gear wheels is arranged for conveying lubricant, with a first of the gear wheels being rotatably supported on a drive shaft and the second of the gear wheels being rotatably supported on an axially shiftable displacement unit for adjusting a variable flow volume, and a control piston axially displaceable in a receptacle of the housing for hydraulically adjusting the position of the displacement unit, with the control piston having a central longitudinal shaft and cylinder rings which include control edges for clearing or closing control channels feeding into the receptacle.
The invention also relates to a control piston for a lubricant pump, in particular as described above.
Lubricant pumps and control pistons of the afore-mentioned type are known from the prior art. In so-called variable capacity lubricant pumps, the feed volume is varied or adjusted by moving a gear wheel of a gear wheel pair. Due to the overlapping zone—seen axially—of the two gear wheels the volume being delivered is determined. The first gear wheel is hereby fixedly mounted on a drive shaft, whereas the second gear wheel is rotatably supported on the displacement unit. The displacement unit can be shifted or moved hydraulically axially in a known manner in order to change the axial position of the second gear wheel and thus the afore-mentioned overlapping zone. An axially movable control piston is provided for hydraulic actuation of the displacement unit and includes a central longitudinal shaft and a plurality of cylinder rings. The diameter of the cylinder rings is hereby selected greater than that of the central longitudinal shaft, with the outer surface areas of the cylinder rings serving as guides in a receptacle of the housing, and with the cylinder rings having control edges which serve to adjust the flow-through cross sections or for clearing or closing control channels, which feed into the receptacle and are fluidly connected to the displacement unit.
As the control piston and the housing have different coefficients of thermal expansion, the known lubricant pumps have the drawback that a precise interaction between the control edges and the control channels feeding into the receptacle cannot always be ensured over a wide operating temperature range, as encountered for example in combustion engines, so that the (axial) position of the second gear wheel does not always correspond to the desired position.